Comment #4 on issue 2738 by [email protected]: Make a distinction between operations and their result
http://code.google.com/p/sympy/issues/detail?id=2738
Let me go back to the beginning. The purpose of this is to kill the usage of Mul(..., evaluate=False). For that, we need to support the functionality that users of it try to achieve: Mul(4, 3, evaluate=False) is used because it prints as "4 * 3". So, what's needed is a way to represent faithfully any syntactically valid mathematical expression, i.e. an AST for mathematical expressions. The Op* objects will be the nodes of this AST. They won't be used for computations, but only for printing, and possibly parsing and meta-programming.
We need OpSub because the string "x - y" is different from "x + -1*y". The first one can be represented as OpSub(x, y) while the second one would be OpAdd(x, OpMul(-1, y))
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